1. Field
The present disclosure relates to a video coding apparatus and a video coding method for coding an input image while dividing the input image into blocks.
2. Description of the Related Art
Nowadays, unified treatment of all pieces of media information such as an image, a sound, and a text becomes general with the progress of multimedia application. Because a digitized image has a huge data amount, an image information compression technology is required to store and transmit the image. At the same time, standardization of the compression technology is also required in order to mutually use the compressed image data. For example, H.261, H.263, and H.264 of ITU-T (International Telecommunication Union Telecommunication Standardization Sector) and MPEG-1, MPEG-3, MPEG-4, and MPEG-4AVC of ISO/IEC (International Organization for Standardization) are well known as a standard for video compression technology. A standardization activity of a next-generation video coding system called HEVC (High Efficiency Video Coding) is currently progressed by a cooperation of ITU-T and ISO/IEC.
In video coding, each coding target picture is divided into blocks which are coding units, and an information amount is compressed by reducing redundancies in temporal and spatial directions in each block. In intra prediction coding aimed at the reduction of the spatial redundancy, a prediction image is generated from pixel information on the surrounding already-coded block to acquire a difference image between the obtained prediction image and a coding target block. In inter-screen prediction coding aimed at the reduction of the temporal redundancy, motion detection and generation of a prediction image are performed by block unit by referring to the picture in which the coding is already completed forward or rearward, and the difference image between the obtained prediction image and a coding target block is acquired. Transform processing such as a discrete cosine transform and quantization processing are performed on the obtained difference image, and a code string is generated using variable-length coding and arithmetic coding, thereby compressing the information amount.
FIG. 1 is a conceptual view illustrating a combination of block sizes defined in the HEVC standard. In the HEVC (ITU-T H.265 (April 2013)) standard, as illustrated in FIG. 1, any size can be selected and used from four kinds of block sizes, namely, 64×64 pixels, 32×32 pixels, 16×16 pixels, and 8×8 pixels as a coding unit (hereinafter, referred to as a “CU (Coding Unit)”).
For example, in the case that a CU size has 32×32 pixels, as illustrated in FIG. 1, any size can be selected and used from eight kinds of block sizes such as 32×32 pixels, 16×32 pixels, and 16×16 pixels as a prediction unit (hereinafter, referred to as a “PU (Prediction Unit)) that is of a unit obtained by dividing the CU. The PU generates a prediction image in the intra prediction coding and the inter-screen prediction coding. For example, a smaller block size is used in the image in which motion of an imaging object is complicated, and a larger block size is used in the image in which the motion of an imaging object is simple, thereby achieving high coding efficiency.
For example, in the case that the CU size has 32×32 pixels, as illustrated in FIG. 1, any size can be selected and used from four kinds of block sizes of 32×32 pixels, 16×16 pixels, 8×8 pixels, and 4×4 pixels as an transform unit (hereinafter, referred to as a “TU (Transform Unit)) that is of a unit obtained by dividing the CU. The TU is used in the transform processing and the quantization processing. For example, a smaller block size is used in the image in which a characteristic varies in a fine range, and a larger block size is used in the image in which the characteristics are identical to each other in a wide range, thereby achieving the high coding efficiency.